iscsi_ibft: filter null v4-mapped v6 addresses

iscsi_ibft: filter null v4-mapped v6 addresses

I've had reports of UEFI platforms failing iSCSI boot in various
configurations, that ended up being caused by network initialization
scripts getting tripped up by unexpected null addresses (0.0.0.0) being
reported for gateways, dhcp servers, and dns servers.

The tianocore EDK2 iSCSI driver generates an iBFT table that always uses
IPv4-mapped IPv6 addresses for the NIC structure fields.  This results
in values that are "not present or not specified" being reported as
::ffff:0.0.0.0 rather than all zeros as specified.

The iscsi_ibft module filters unspecified fields from the iBFT from
sysfs, preventing userspace from using invalid values and making it easy
to check for the presence of a value.  This currently fails in regard to
these mapped null addresses.

In order to remain consistent with how the iBFT information is exposed,
we should accommodate the behavior of the tianocore iSCSI driver as it's
already in the wild in a large number of servers.

Tested under qemu using an OVMF build of tianocore EDK2.

Signed-off-by: Chris Leech <cleech@redhat.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>